Fluoroelastomers have elastomeric properties (high elongation at break) and show high resistance to heat and chemicals. They can be prepared by curing amorphous fluoropolymers. The polymer architecture resulting from the curing reaction provides the elastomeric properties. Fluoroelastomers have been widely used as raw materials and major component of elastomer compositions. Elastomer compositions have been used in producing seals (such as, for example, O-rings), coatings, laminates and hoses, in particular in containers for medical applications or in applications in the automotive or aircraft industry where resistance to fuel is desired. In many such applications it is desired that the fluoroelastomers retain their elastomeric properties over a wide temperature range. In particular when used in aircrafts, motorcrafts and watercrafts or in articles used in cryogenic applications or conditions, fluoroelastomers are required to be sufficiently flexible or elastomeric also at temperatures below −50° C. or even below −70° C. At temperatures above the glass transition temperature (Tg) sufficient thermal energy is available to allow motion of the segments in the main backbone chain of the polymer to provide sufficient flexibility. Thus for being applicable in low temperature applications, fluoroelastomers having a very low Tg are required.
Fluoroelastomers for use in fluoroelastomer compositions of high chemical and temperature resistance and good mechanical properties may be prepared by curing a system comprising olefinically unsaturated perfluorovinyl ethers and cure site monomers. Typically, glass transition temperatures of about −30° C. can be obtained by such systems.
A different approach has relied on the formation of fluoroelastomers containing polytriazines by reacting functionalised fluoropolyethers with suitable catalysts. For example, U.S. Pat. No. 5,693,748 reports a method wherein a functionalised perfluoropolyether was converted first into a polyimidoylamidine. The resulting linear polyimidoylamidine polymer was then treated with acylating agents to convert the imidoylamidine groups into triazines. By using functionalised acylating agents polytriazines were generated carrying pendant nitrile groups that were cured using ammonia as curing catalyst. The resulting polymer was reported to have a Tg of −45° C. However, the cured polymers were brittle and thus not usable as elastomers when the polymer unit bearing the pendant nitrile group had a molecular weight of less than about 25,000 g/mole per nitrile group.
Although a wide range of fluoropolymers of different chemical composition is known, commercial fluoroelastomer compositions having despite their high elongation at break also good mechanical properties like high tensile strength appear to have glass transition temperatures of only around −20° C. (compare A van Cleef, in Modern Fluoropolymers, John Scheirs ed., John Wiley & Sons, 1997, pages 597-613).
There has been a continuous need to provide fluoroelastomers having a glass transition temperature below −20° C., preferably below −50° C. or even lower. It is desirable that the flexibility at low temperatures is not compromised by reduced mechanical properties like tensile strength. Preferably, the fluoroelastomers can be prepared by curing in closed molds to reduce the exposure of operators to fumes generated by the curing process.